Free PC Games

Spelunky is a roguelike inspired platformer made by Derek Yu. The Xbox remake heavily expands upon the original game in every single way. The PC version has also a lot of different mods which you can download. If you're going to download the original PC version, then I highly recommend you download the unofficial 1.3 version of the game. It fixes a lot of bugs and it also runs much smoother.

Eternal Daughter is a metroidvania released by Derek Yu.

Treasure Adventure Game. You can even add the game to your GOG-catalogue.

Hero is the first game in the "Hero" series. It's only in black and white and it's inspired by old games from the Commodore 64 and Atari era.

Hero Core is a minimalistic metroidvania. This is the second game in the "Hero" series and it's a full fledged metroidvania (compared to the first game). It's available on both PC and Mac.

Hydra Castle Labyrinth is a Japanese metroidvania developed by E. Hashimoto (aka. Buster). Here are some mirrors if the original link doesn't work: http://jayisgames.com/archives/2012/01/hydra_castle_labyrinth.php or http://vsrecommendedgames.wikia.com/wiki/Hydra_Castle_Labyrinth.